For consumer mobile devices and automotive audio devices, customers expect good sound quality, high output power, low distortion and long-term battery life. Class-D amplifiers are suitable for this type of application.
Class D amplifiers use nonlinear amplification that involves switching of the output between discrete voltage levels.
The benefit of class-D amplification over linear amplification is that there is less power dissipation or higher efficiency in the amplifier, for example >90%, compared to linear amplifier types such as class-A, -AB or -B. A class-AB amplifier has a typical efficiency of 78.5% for a sine wave and about 25% for music signals.
Most present class-D amplifiers comprise a modulator, a power stage and a low pass filter at the output. The modulator converts the input signal into a Pulse Width Modulated (PWM) signal that is used to drive the power stage. The low frequency content of the PWM signal represents the desired output signal of the class-D amplifier and a low pass filter is used to reconstruct the desired output signal.
Normally, a low loss second or higher order LC filter is used, but the use of an inductor coil is undesirable because it is expensive, bulky and non-linear. Depending on the characteristics of the loudspeaker used, the efficiency of a 2-level filterless class-D amplifier is reduced compared to a filtered class-D amplifier because the load has to handle the high frequency energy of the PWM signal.
FIG. 1 shows the load current of a filtered and non-filtered PWM signal and shows a PWM voltage generated at the output of a CMOS pull-up and pull-down stage 10. The square wave PWM voltage 12 across the load results in a square wave load current in the absence of any filtering (load current 14), whereas the current is smoothed (load current 16) by the inductor when an inductive filter is used.
The efficiency of a filterless or filtered class-D system can be improved if multiple output levels are used, instead of a two-level system as shown in FIG. 1.
There are several ways to generate PWM signals. They can be categorized as two main approaches:
1. Fixed carrier PWM modulation
Carrier-based PWM is a well-known method. A fixed frequency is used to sample the input signal. Two main types of carrier-based PWM are applicable: natural sampling (NPWM) and uniform sampling (UPWM).
2. Variable carrier PWM modulation
Self-oscillation is the most commonly known principle used by a variable carrier modulator.
FIG. 2 shows a 2-level fixed carrier pulse width modulation scheme.
In general a fixed carrier N-level PWM modulator is preferred because the fixed carrier signals result in a known frequency spectrum of the output signal.
In FIG. 2, a sinusoidal input VS is compared with a symmetrical triangular waveform VT by a comparator 20. The crossing point of the two input signals determines the timing at which switching between the two levels of the output occurs.
The ratio between the amplitude of the (sinusoidal) input signal, VS, and amplitude of the triangular wave, VT, is called the modulation depth, Δ.
            V      INPUT        =                  V        S            ·              cos        ⁡                  (                                    ω              S                        ⁢            t                    )                          Δ    =                  V        S                    V        T            
The 2-level PWM signal, F(t), obtained from a sinusoidal input signal that is compared with a reference triangle can be expanded in a Fourier series:
            F      ⁡              (        t        )              =                  Δ        ⁢                                  ⁢                  cos          ⁡                      (            y            )                              +              2        ⁢                              ∑                          m              =              1                        ∞                    ⁢                                          ⁢                                                                      J                  0                                ⁡                                  (                                      m                    ⁢                                                                                  ⁢                    π                    ⁢                                          Δ                      2                                                        )                                                                              m                  ⁢                                                                          ⁢                  π                                2                                      ⁢                          sin              ⁡                              (                                                      m                    ⁢                                                                                  ⁢                    π                                    2                                )                                      ⁢                          cos              ⁡                              (                mx                )                                                        +              2        ⁢                              ∑                          m              =              1                        ∞                    ⁢                                    ∑                              n                =                                  ±                  1                                            ∞                        ⁢                                                                                J                    n                                    ⁡                                      (                                          m                      ⁢                                                                                          ⁢                      π                      ⁢                                              Δ                        2                                                              )                                                                                        m                    ⁢                                                                                  ⁢                    π                                    2                                            ⁢                              sin                ⁡                                  (                                                                                    (                                                  m                          +                          n                                                )                                            ⁢                                                                                          ⁢                      π                                        2                                    )                                            ⁢                              cos                ⁡                                  (                                      mx                    +                    ny                                    )                                                                                            Δ                                          Modulation            ⁢                                                  ⁢            depth                    ,                      Δ            ∈                          [                              0                ,                1                            ]                                                                    x          =                                    ω              C                        ⁢            t                                                Carrier          ⁢                                          ⁢          signal          ⁢                                          ⁢          frequency                                              y          =                                    ω              S                        ⁢            t                                                Audio          ⁢                                          ⁢          signal          ⁢                                          ⁢          frequency                                              J          N                                      Bessel          ⁢                                          ⁢          function          ⁢                                          ⁢          of          ⁢                                          ⁢          order          ⁢                                          ⁢          N                                    n                              Audio          ⁢                                          ⁢          Signal          ⁢                                          ⁢          harmonics          ⁢                                          ⁢          index                                    m                              Carrier          ⁢                                          ⁢          Signal          ⁢                                          ⁢          harmonics          ⁢                                          ⁢          index                    
A multi-level PWM signal, FN(t), is able to represent the desired output signal more accurately. A multi-level PWM signal can also be expanded using a Fourier series:
                    ⁢                            F          N                ⁡                  (          t          )                    =                        Δ          ⁢                                          ⁢                      cos            ⁡                          (              y              )                                      +                  2          ⁢                                    ∑                              m                ∈                                  {                                      N                    ,                                          2                      ⁢                                                                                          ⁢                      N                                        ,                                                                                  ⁢                                          3                      ⁢                                                                                          ⁢                      N                                        ,                                                                                  ⁢                    …                                    ⁢                                                                          }                                                      ⁢                                                  ⁢                                                                                J                    0                                    ⁡                                      (                                          m                      ⁢                                                                                          ⁢                      π                      ⁢                                              Δ                        2                                                              )                                                                                        m                    ⁢                                                                                  ⁢                    π                                    2                                            ⁢                              sin                ⁡                                  (                                                            m                      ⁢                                                                                          ⁢                      π                                        2                                    )                                            ⁢                              cos                ⁡                                  (                  mx                  )                                                                    +                  2          ⁢                                    ∑                              m                ∈                                  {                                      N                    ,                                          2                      ⁢                                                                                          ⁢                      N                                        ,                                                                                  ⁢                                          3                      ⁢                                                                                          ⁢                      N                                        ,                                                                                  ⁢                    …                                    ⁢                                                                          }                                                      ⁢                                          ∑                                  n                  =                                      ±                    1                                                  ∞                            ⁢                                                                                          J                      n                                        ⁡                                          (                                              m                        ⁢                                                                                                  ⁢                        π                        ⁢                                                  Δ                          2                                                                    )                                                                                                  m                      ⁢                                                                                          ⁢                      π                                        2                                                  ⁢                                  sin                  ⁡                                      (                                                                                            (                                                      m                            +                            n                                                    )                                                ⁢                                                                                                  ⁢                        π                                            2                                        )                                                  ⁢                                  cos                  ⁡                                      (                                          mx                      +                      ny                                        )                                                                                                              Δ                                          Modulation            ⁢                                                  ⁢            depth                    ,                      Δ            ∈                          [                              0                ,                1                            ]                                                                    x          =                                    ω              C                        ⁢            t                                                Carrier          ⁢                                          ⁢          signal          ⁢                                          ⁢          frequency                                              y          =                                    ω              S                        ⁢            t                                                Audio          ⁢                                          ⁢          signal          ⁢                                          ⁢          frequency                                              J          N                                      Bessel          ⁢                                          ⁢          function          ⁢                                          ⁢          of          ⁢                                          ⁢          order          ⁢                                          ⁢          N                                    n                              Audio          ⁢                                          ⁢          Signal          ⁢                                          ⁢          harmonics          ⁢                                          ⁢          index                                    m                              Carrier          ⁢                                          ⁢          Signal          ⁢                                          ⁢          harmonics          ⁢                                          ⁢          index                                    N                              Number          ⁢                                          ⁢          of          ⁢                                          ⁢          PWM          ⁢                                          ⁢          levels                    
In general, the generation of an N-level PWM signal requires (N−1) phase shifted reference carrier signals. The phase shift of the different N−1 carriers is set as:
            Φ      P        =          p      ⁢                        2          ⁢          π                          N          -          1                      ,          ⁢      p    ∈          [              0        ,                  N          -          2                    ]      
This invention relates to multi-level class-D amplifiers.
It has been proposed to provide a five voltage level class-D system built-up by using two 3-level power stages configured in a bridge tied load configuration.
This configuration is shown in FIG. 3.
Each 3-level stage comprises three transistors each connected between a respective power supply and a common node. The first 3-level power stage P (P=plus) comprises transistors MHP (HP=high, plus) connected between the node 30 and a high voltage rail VDD, MMP (MP=mid, plus) connected between the node 30 and a middle voltage VDD/2, and MLP (LP=low, plus) connected between the node 30 and a low voltage rail GND.
The second 3-level power stage M (M=minus) comprises transistors MHM (HM=high, minus) connected between the node 32 and the high voltage rail VDD, MMM (MM=mid, minus) connected between the node 32 and the middle voltage VDD/2, and MLM (LM=low, minus) connected between the node 32 and the low voltage rail GND.
This uses a pair of 3-level power stages. It is noted that a 2-level class-D power stage can be used for a three level class-D amplifier by choosing another modulation method, BD. BD is a 3 level modulation instead of 2 level AD modulation, which is configured in bridge tied load configuration. The difference between AD and BD mode is visualized in FIG. 4a, which shows a simplified representation of AD and BD modulation.
In an AD modulation scheme, inverted reference signals are applied to two comparators, which receive the differential input signals. This means the crossing points of the two comparators correspond in time. In a BD modulation scheme, the same reference signals are applied to the two comparators with the effect that the crossing points arise at different times. These differences enable a three level signal to be derived.
FIG. 4b shows timing diagrams for AD modulation and FIG. 4c shows timing diagrams for BD modulation.
Power stage errors introduced by the switching power stage cause distortion, and they can be categorized into Pulse Timing Errors [PTE] and Pulse Amplitude Errors [PAE].
The essential sources of Pulse Timing Errors are:
(i) Difference in delay between turn-on and turn-off. The delays from turn-on or turn-off to the actual PWM level voltage transition are different. The delays depend on various parameters in the Power MOSFET physics and driver hardware and are complex to analyze and impossible to correct directly by tuning.
(ii) Deadtime. This is the blanking delay between a turn-off and the following turn-on.
(iii) Current dependent rise and fall times. The slopes of the edges of the PWM signal are different in rising and falling edges, and they depend on the present current through the power switches.
(iv) Overshoot and ringing of the edges of the PWM pulse. The surface of the PWM pulse is changed when overshoot and ringing are added to the required PWM pulse.
Pulse Amplitude Errors are constituted by:
(i) Power supply perturbations. Since the power supply level directly generates the PWM waveform, any power supply variation will influence the modulated audio signal.
(ii) Non-zero power switch impedance. The impedance of the switch during the on period is not zero. Non-zero power switch impedance will not necessarily lead to harmonic distortion or extra noise. Errors do result if the switching impedances of the three power transistors are different.
The control of a class D amplifier can use a feedforward approach or a feedback approach. Applying feedback minimizes the errors outlined above.
However, the implementation of a feedback control loop for a multi-level class-D amplifier is complicated, and feedforward control systems are generally used.